Hi @Embedded_novice,
According to the FlexCAN MCAL user manual, in case of HRH, HwObjectCount is number of
elements in the hardware FIFO.
Regarding your question, yes, CAN hardware objects defined as a PDU buffer. The CAN RAM block can store message buffers in different configurations of sizes. According to the S32K3XX reference manual, the RAM block can store 32 messages of 8 bytes, 21 messages of 16 bytes, 12 messages of 32 bytes or 7 messages of 64 bytes. So, it is based on MB.
The CAN Interface (CanIf) is found between the low-level CAN drivers and the upper communication service layers from the AUTOSAR stack. It is not related to the Mailbox concept. The mailbox system is composed of a set of message buffers (MB) that store configuration and control data, time stamp, message ID, and data (see Message buffer structure).

You can read the following article to know more about the CAN module: Communicating over the CAN Bus - NXP Community.
Hope this helps.